1. Field of the Invention
The present invention relates to gas analysis via spectroscopy, and more specifically, it relates to the use of infrared absorption and Raman scattering spectroscopy and assays for the qualitative and quantitative analysis of gases.
2. Description of Related Art
Gas detection utilizing Raman spectroscopy has previously been demonstrated using a solid-core optical fiber probe, typically comprised of one or more optical fibers for excitation and signal collection. These probes provide a conduit (e.g., light pipe) for exciting a gas or a gas mixture under test with an excitation wavelength(s) of light that generates Stokes and/or anti-Stokes scattering, part of which is collected by a signal collection fiber(s) and provided to a spectrometer. Wavelength cross-sections for gas species have been documented. The main technical problem with this approach is the low signals that are generated from the gas analyte because of the low Raman scattering cross-sections and potential background interferences. This severely limits the sensitivity level, making gas detection at low ppm levels difficult. Studies have used a multi-pass cell or cavity, as well as optical fiber multi-pass cells, to increase the interaction path length between the gas molecules and excitation photons, which improves the limits of detection. However, implementing this cavity design in the field for gas detection is difficult because it requires delicate and precise alignment, a small size, and lacks long-term robustness and rigidity.
Chemical detection using optical spectroscopic techniques frequently requires significant enhancement of the signals in order to achieve detection limits of parts per million (ppm) levels. As mentioned above, one standard approach is the use of multi-pass cavities that maximize the interaction length between the chemical analyte and the excitation light. The cavity usually consists of two mirrors that reflect the light back and forth. Two parabolic mirrors can be used to focus multiple reflections to the same point. Field deployment of these devices is difficult because of the sensitivity of their optical alignment. Due to the limitations and difficulties of using multi-pass configurations in the field, utilizing optical fiber is frequently more suitable under these conditions. Work in this area has largely focused on solid-core optical fiber probes (such as e.g. Raman gas sensing with fiber optic probes). Fiber based IR absorption spectroscopy for trace gas detection has previously been demonstrated. As in the Raman techniques described above, the fiber geometries used for IR absorption spectroscopy are likewise usually solid core based.